Gallium nitride materials include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and aluminum indium gallium nitride (AlInGaN). These materials are semiconductor compounds that have a relatively wide, direct bandgap which permits highly energetic electronic transitions to occur. Such electronic transitions can result in gallium nitride materials having a number of attractive properties including the ability to efficiently emit blue light, the ability to transmit signals at high frequency, and others. Accordingly, gallium nitride materials are being widely investigated in many microelectronic applications such as transistors, field emitters, sensors (e.g., chemical, biological, and radiological in nature), and optoelectronic devices.
In general, the crystal structure of a gallium nitride material includes an arrangement of nitrogen atoms, (which also may be replaced in certain sites by other Group V elements such as B, As, P, Sb), and gallium atoms (which also may be replaced in certain sites by other GroupIII metals such as Al, In) in a (0001) crystallographic orientation. The nitrogen atoms are typically arranged in a series of planes in the structure; and, the gallium atoms are also arranged in a series of different planes. As disclosed herein, the “face” of a crystal structure is defined by the atomic bonding arrangement within a plane of atoms (e.g., a plane of nitrogen atoms, or a plane of gallium atoms). It should be understood that in this context, a “plane” of atoms may generally approximate a planar arrangement but need not be arranged in an ideal plane.
As shown in FIGS. 1A and 1B, if a plane of nitrogen atoms 6 is oriented with bonds along the c-axis at the topside of the crystal structure, the plane is referred to as an N-face 2; and, if a plane of gallium atoms 8 is oriented with bonds along the c-axis at the topside of the crystal structure, the plane is referred to as a Ga-face.
Thermodynamic considerations typically result in the top surface of a gallium nitride material layer having a gallium-face. Thus, features (e.g., layer(s)) formed on top of such gallium nitride material layers are formed on the gallium-face.